The invention relates generally to temperature monitoring transistors within a semiconductor circuit, and deals more particularly with a semiconductor layout of a single or plurality of transistors which monitor the temperature of a load transistor.
The following patent applications filed herewith have a common Detailed Description:
Ser. No. 07/782,211, filed on Oct. 24, 1991, by D. J. Ashley and M. K. Demoor, entitled "Temperature Compensated Over Current and Under Current Detector", now U.S. Pat. No. 5,245,261.
Ser. No. 07/782,218, filed on Oct. 24, 1991, by D. J. Ashley, M. K. Demoor and P. W. Graf, entitled "Temperature Compensated Circuit For Controlling Load Current", now U.S. Pat. No. 5,237,262.
Some solenoids, motors and other loads require precise control of their drive current. For example, a precision print hammer can be driven by a solenoid and require precise drive current so that the speed and force of impact of the print hammer meet tight specifications. The drive current can be affected by several factors including temperature. Much of the temperature affects are due to heating of a load transistor (which passes the drive current) from the very current required to drive the load, and this heating cannot usually be avoided. The heating of the load transistor increases an "on-resistance" of the load transistor, which on-resistance is a characteristic of the drain to source path in an FET transistor or the collector to emitter path in a bipolar transistor. The increased on-resistance will naturally tend to reduce the load current for a constant drive voltage so that the drive voltage must be adjusted to maintain the load current within a predetermined range. Other factors can also affect the load current.
As a result of all affects on the load current, some form of feedback is often utilized to continuously control the drive voltage to maintain the load current within the predetermined range. For example, a small resistor has been placed in series with the load, and the voltage across the resistor used to monitor the drive current. This technique has the disadvantages of power dissipation in the series resistor, and imprecision due to the variation in the resistance of the series resistor itself with changing temperature. A more recent technique utilizes a "pilot" transistor which is a scaled version of the load transistor. For example, the load transistor is made of hundreds of thousands of identical transistors connected in parallel to handle a sizable load current and the corresponding pilot transistor is a unitary transistor having the size of hundreds of the same transistors connected in parallel. The pilot transistor and the load transistor are both integrated into the same "chip".
Note, that in a "drain pilot" design, the drain pilot transistor does not actually pass any of the load current, and the heating of the pilot transistor is due primarily to the heat conduction through the substrate or associated metal layer of the common chip. Nevertheless, as the load transistor heats-up due to the load current, the drain pilot transistor also heats-up and the on-resistance of the pilot transistor changes proportionally to the on-resistance of the load transistor. A constant current source feeds the drain pilot transistor on-resistance and therefore, develops a voltage which is proportional to the desired load current at any temperature. (This current is small to avoid significant heating of the pilot transistor.)
Because the on-resistance of the drain pilot transistor tracks the on-resistance of the load transistor, the voltage developed across the drain pilot transistor is an accurate reference to compare to the voltage developed across the load transistor. When the voltage developed across the load transistor is greater than the reference, then the power supply which provides the drive voltage is disconnected from the load for a predetermined period to cause the drive current to decrease. Then, the power supply is re-connected to increase the load current. This cycle is repeated for the duration of the drive current to maintain the drive current within the predetermined range. The result is a reasonably accurate control of the drive current considering all affects on the load current including temperature.
Previously it was known to locate the drain pilot transistor adjacent to the load transistor near an edge of the load transistor, and this has provided reasonably good temperature compensation. However, the temperature of the pilot transistor has not tracked the temperature of the load transistor with sufficient accuracy for some applications. Also, it should be noted that the temperature of the load transistor is not uniform; the load transistor is large and comprises hundreds of thousands of transistors. In a typical load transistor, the individual transistors near the center of the device are somewhat hotter than the individual transistors near the edges. All these individual transistors with their different on-resistances are connected in parallel to yield a single composite on-resistance which affects the load current.
In a "source pilot" design, the source pilot transistor is also a scaled model of the load transistor and is integrated into the same chip as the load transistor, but in contrast to a drain pilot, actually draws some of the load current. The magnitude of the current drawn by the source pilot transistor is monitored and used to control the load current.
Some electrical circuits which are the subject of copending patent applications entitled filed same day herewith by and filed same day herewith by require more than one drain pilot transistor to track the temperature of the same load transistor.
A general object of the present invention is to provide a semiconductor layout of a load transistor, and a single or plurality of associated pilot transistors which accurately track the temperature of the load transistor.
Another general object of the present invention is to provide a semiconductor layout of the foregoing type in which plural pilot transistors accurately track the temperature of each other.
Still another general object of the present invention is to provide a semiconductor layout of the foregoing type in which plural pilot transistors minimize impact on the layout of the load transistor.